A "Missing profile" popup alert is displayed when a relative path is specified to the "-profile" Firefox command line option. This works OK if the profile path is an absolute path. This works: $ arch -i386 /Applications/Firefox-Aurora.app/Contents/MacOS/firefox-bin -profile /Users/hop/Library/Application\ Support/Firefox/Profiles/xq1q5l07.XXX/ This doesn't work: $ cd /Users/hop $ arch -i386 /Applications/Firefox-Aurora.app/Contents/MacOS/firefox-bin -profile Library/Application\ Support/Firefox/Profiles/xq1q5l07.XXX/ Being able to specify relative paths is useful for example when running tests. Firefox accepts both absolute and relative paths on Linux and Windows, from Firefox 5.0 up to Aurora (7.0a2), which is what made conclude of a buggy behavior.
I have done more tests and can say now that this regression appeared on Mac between Firefox 3.6 and Firefox 4.0: Firefox 3.6 was accepting relative path profiles, while starting with Firefox 4.0 relative path profiles are not accepted anymore.
This bug is filed in a bugzilla component related to pre-Firefox code which no longer exists. I believe it is no longer relevant and I am therefore closing it INCOMPLETE. If you believe that this bug is still valid and needs to be fixed, please reopen it and move it to the Toolkit:Startup and Profile System product/component.